For mobile data communications, layered communication networks may be deployed as e.g. described by the 3GPP specification (3rd Generation Partnership Project). The 3GPP specification comprises three distinct layers: an application layer, a network control layer and a connectivity layer. The application layer supports end-user applications and may be implemented in mobile stations or application servers in the network. The application layer interfaces with the network layer to enable designing and implementing different services and applications. The network control layer supports communicating services across e.g. different types of networks such as circuit-switched domain networks based on the GSM standard using e.g. an ISDN-related technology or packet-switched networks employing e.g. the GPRS (General Packet Radio Service) technology. The connectivity layer is a transport layer capable of transporting any type of service via e.g. voice, data and multimedia streams.
The 3GPP specification defines the Long Term Evolution (LTE) as an access network technology. For delivering multimedia services across an access network, the 3GPP specification defines the IP Multimedia Subsystem (IMS). The IMS comprises a service layer forming an application layer, a control and connectivity layer forming a network control layer, and an access layer forming a connectivity layer. The control and connectivity layer comprises call session control functions (CSCF) forming central entities for Session Initiation Protocol (SIP) signalling. According to the IMS, an application server is provided for hosting and executing services for user entities. An example of an application server is the Multimedia Telephony Service server (MMTeI) offering multimedia communications such as voice. Another example of an application server is the Voice Call Continuity Server (VCC).
According to the IMS, the application server interfaces with the Serving CSCF (S-CSCF) using the Session Initiation Protocol (SIP). The S-CSCF transmits a SIP invite message towards the application server in order to establish a communication link towards a user entity. The SIP invite message addresses the user entity using a public identification such as Mobile Subscriber Integrated Services Digital Network Number (MSISDN). If the user entity is available, then the application server returns the SIP invite message towards the S-CSCF. The S-CSCF forwards the received SIP invite message towards the user entity. However, the public identification may also address a set of user entities, each having an own Subscriber Identity Module (SIM) of a Multi-SIM arrangement. In that case, the S-CSCF generates a plurality of copies of the received SIP invite message from the application server, and transmits the plurality of copies towards the set of user entities, which is also referred to as forking.